It is well known to separate blood into its component parts by centrifugation, and to provide a barrier between the lighter liquid phase, e.g., the serum or plasma, and the heavier formed phase, e.g., the cells and/or fibrin, so as to physically separate the lighter phase from the heavier phase in the centrifuged blood sample. The blood to be tested may be collected in a blood collection container, and is thereafter centrifuged to separate the phases. In some cases, the barrier will be fixed in the tube, as shown in U.S. Pat. No. 3,945,928, and others; and in other cases the barrier will be movable in the tube during centrifugation, as shown in U.S. Pat. No. 5,271,852, and others. The barrier is typically a solid device, or a gel, In some cases the barrier may be positioned in the tube after the blood sample has been drawn, as shown in U.S. Pat. No. 4,057,499, for example; and in other cases the barrier may be positioned in the tube prior to drawing the blood sample, as shown in U.S. Pat. No. 3,779,383, among others.
There have been a great many solutions to the problem of how to separate the formed components from the unformed components in a centrifuged blood sample, but all suffer from various drawbacks. For example, it is undesirable to open the tube or perform any manipulations which might expose the operator to the blood sample, therefore it is highly desirable to have the barrier in the tube prior to drawing the blood sample. When a fixed barrier is used, care must be taken to locate the barrier in the tube in a position which will ensure that all of the formed component fraction of the blood sample will gravitate to a location that is below the fixed barrier during centrifugation. This frequently results in some of the non-formed component fraction also ending up beneath the barrier, and thus being lost to the clinician, a result which is undesirable.
When a solid barrier is preloaded into the tube prior to drawing the sample, there have been several approaches for solving the problem of how to separate the solid phase from the liquid phase in the blood sample by centrifugation, it being obvious that one must be able to get the solid phase components, i.e., essentially the cells, past the solid barrier. One solution suggested in the prior art is to provide the solid barrier with a flexible skirt which partially collapses during the centrifugation so as to allow the solid phase and liquid phase fractions to pass by the exterior of the barrier. Another solution is to place the solid barrier on the bottom of the tube prior to drawing the sample into the tube. In this case, the barrier may include an internal opening which acts as a check valve through which the heavier phase is intended to pass during centrifugation so as to lift the barrier to a location in the tube which is between the heavier and lighter phases of the centrifuged sample. Alternatively, the barrier may include an outer part which will contract when subjected to the pressure developed in the sample during centrifugation. The heavier phase is intended to gravitate around the barrier so as to lift the barrier to the proper location in the tube. These solutions are relatively complex, and are not readily usable with clotted blood because the clots cannot readily bypass the solid barrier in the tube during centrifugation.
The separating barrier must be inert, i.e., it must not add to or subtract from the chemistry of the serum/plasma phase. One problem encountered with gel barrier materials is that they may absorb some sample additives, e.g., therapeutic drugs or other additives placed in the tube and necessary for proper sample analysis; and also may also spawn or generate gel particles which can clog instruments used to analyze the serum/plasma phase.
It would be highly desirable to provide a solid, inert barrier which can be preloaded into a blood drawing tube; which will not interfere with drawing of the blood sample; and which will settle onto the heavy phase-lighter phase interface during centrifugation of the sample. It would be highly desirable to provide such a barrier member which can be used with both anti coagulated, and non-anti coagulated blood samples.